1. Field of the Invention
The present invention relates to a pacer system which is adapted to alter the rate of pacing pulses delivered by an artificial pacemaker to a heart while an individual is exercising utilizing ejection time to obtain a required cardiac output.
2. Description of the Prior Art
Heretofore patients with heart dysfunctions or heart disease such as sinus node disease have been able to live a relatively normal life with the implantation of an artificial pacemaker often referred to as a pacer. However, such pacers have not always been able to mimic the response of a normal healthy heart. A normal heart responds to exercise and stress by increasing cardiac output through increased heart rate or stroke volume.
In this respect, patients with sinus node disease have lost the ability to increase heart rate with exercise. Accordingly, it has become a goal of optimal pacing to provide exercise responsiveness in a pacer by sensing the need for increased cardiac output.
With a view toward obtaining this goal, a number of pacemakers have been proposed for indirectly sensing the need for increased heart rate by sensing P-waves, nerve impulses, Q-T interval, pH, oxygen saturation, respiratory rate, stroke volume, motion, atrial pressure and temperature.
A P-wave triggered artificial pacemaker adapted to be exercise responsive by responding to average atrial rate has been proposed in the Knudson & Amundson U.S. Pat. No. 4,313,442.
An artificial pacemaker responsive to changes in the Q-T interval is proposed in the Rickards U.S. Pat. No. 4,228,803.
The Heilman et al. U.S. Pat. No. 4,303,075 discloses a method and apparatus for maximizing stroke volume through AV pacing using an implanted cardioverter/pacer which is programmed with an AV delay tailored to the particular patient. The mechanism detects and processes the impedance of the heart across two electrodes in contact with heart muscle during each heart cycle and uses the changes from cycle to cycle to trigger the issuance of pulses from the pacer depending on the direction of the impedance changes to maximize stroke volume of the heart, which is proportional to the change in value of impedance between the minimum and maximum detected impedance per heart cycle.
The Funke U.S. Pat. No. 4,312,355 discloses a dual pace-dual sense cardiac pacer which is able to stimulate the atrium and/or the ventricle and which is able to entrain the ventricle, when the atrial rate increases, while preventing bradycardic episodes. The pacer action is triggered by detection of naturally occurring atrial and ventricular action or pulses within a predetermined time period.
The Roline U.S. Pat. No. 4,363,325 discloses a multiple mode pacer activated to switch modes relative to heart rate thereby preventing atrial bradycardia. This is achieved by mode switching of the pacer from an atrial synchronous mode to a ventricular inhibited mode. Such switch of modes is actuated when no atrial activity is sensed within a preset escape interval referred to as a hysteresis period. Reversal of the mode back to the atrial synchronous mode from the ventricular inhibited mode is actuated in response to a detected atrial rate which is higher than a preset, lower, ventricular rate. With this mode switching, the ventricle will not be stimulated twice in quick succession, which overstimulation could cause atrial bradycardia.
A proposal for placing electrodes on the Hering's nerve that extends from receptors in the sinus and glomus carotids is disclosed in the Gonzalez U.S. Pat. No. 4,201,219.
Sensors for sensing blood pH are proposed in the Alcidi U.S. Pat. No. 4,009,721 and the Mauer et al U.S. Pat. No. 4,252,124. Alcidi controls a pacer relative to blood pH.
In the Bornzin U.S. Pat. No. 4,467,807 molecular oxygen is sensed with an oxygen sensor, preferably of the type as disclosed in the Wirtzfeld et al U.S. Pat. Nos. 4,202,339 and 4,299,820. The Wirtzfeld et al patents teach measuring of oxygen saturation of blobd using optical techniques. The transmissiveness of light through blood is used by Wirtzfeld et al to measure oxygen concentration. Bornzin teaches using such measurements for controlling the pacing of a heart.
An artificial pacemaker that senses oxygen saturation at the tip of a catheter in the right ventricle is proposed in the Wirtzfeld et al U.S. Pat. No. 4,202,339.
Another artificial cardiac pacemaker which increases pacing rate in accordance with an increase in respiration rate is proposed in the Krasner U.S. Pat. No. 3,593,718.
Pacers for sensing motion or mechanical activity are proposed in the Dahl U.S. Pat. No. 4,240,132 and the Anderson et al U.S. Pat. No. 4,428,378.
The Denniston III U.S. Pat. No. 3,815,611 discloses an apparatus which detects muscle contractions through impedance measurement. The device includes an elastomer body whose impedance changes when flexed. The elastomer body is positioned adjacent a muscle such as a heart muscle such that when the muscle contracts, the elastomer body is flexed to provide a change in impedance to a bias voltage supplied thereto. Such electrical signal can be used to control a pulse generator to generate a pulse when a specified period of time has elapsed since the latest heart activity was sensed by the elastomer body.
Another artificial cardiac pacemaker responsive to exercise by sensing venous blood temperature in the right ventricle of the heart is proposed in the Cook et al U.S. Pat. No. 4,436,092.
In a healthy heart, the sympathetic nervous system activates the sinus node to increase its rate. Secondly, it activates the heart to increase its dynamic contraction during exercise. The first activation is lost in patients with sinus node disease but the second phenomenon of dynamic contraction still exists. This means that exercise modified increased sympathetic activity can be detected by measuring the changes in the ejection time in the right ventricle. As will be described in greater detail hereinafter, the apparatus and method of the present invention utilizes changes in the ejection time in the right ventricle to control the pacing rate of an implanted artificial pacemaker.
Heretofore it has been proposed in the Cohen U.S. Pat. No. 3,358,690 to sense pressure in the right atrium and to utilize the pressure sensed to control pacing of an electrode in the right ventricle.
Also, the Zacouto U.S. Pat. No. 3,857,399 discloses, in FIG. 19 thereof, a pressure sensor that measures either left ventricular pressure or intramyocardial pressure. One sensor is located in the myocardium or septum and the other sensor is located in the left ventricle. Apparently, the pacer coupled to these sensors responds to average base pressure over relatively long periods of time and the pacer system disclosed therein appears to be static and slowly responsive to exercise.
The Sjostrand et al. U.S. Pat. No. 3,650,277 discloses a system for reducing and controlling the blood pressure of a hypertensive patient by electrical stimulation of the carotid sinus nerves, one of the baroreceptor centers of the body. The system incorporates a pressure transducer which is connected to or applied to an artery of a patient and provides electrical signals substantially representing the instantaneous arterial blood pressure of a patient. Upon calculation of a mean arterial pressure, the system is utilized to provide a series of electrical pulses having a predetermined frequency, magnitude and amplitude through an afferent nerve, such as the carotid sinus nerve, to the heart to mimic pulses to the heart occurring naturally in patients having normal blood pressure. These pulses are provided during the first portion of each heart cycle to take over the function of controlling blood pressure that is usually provided by normally functioning baroreceptors in patients who are not hypertensive.
In the field of artificial hearts, it has been proposed in the Purdy U.S. Pat. No. 3,828,371 directed to a self contained artifical heart, to control the operation of an artificial heart relative to the sensing of atrial pressure.
Further, there is disclosed in U.S.S.R. Inventor's Certificate No. 858,843 a device for controlling a prosthesis of the heart which replaces a natural heart and pressure in the atrium of the heart.
The apparatus of the present invention can utilize a pressure sensor for sensing right ventricular pressure changes for determining ejection time as well as changes in ejection time. The pacer rate is adjusted relative to changes in ejection time. Further, a number of different pressure sensors, including piezoresistive pressure sensors, can be used to sense the right ventricular pressure. See for example the pressure sensors and transducers disclosed in the following U.S. patents:
______________________________________ U.S. PAT. NO. PATENTEE ______________________________________ 2,634,721 Greenwood, Jr. 2,976,865 Shipley 3,088,323 Welkowitz et al 3.038,465 Allard et al 3,294,988 Packard 3,563,245 McLean 3,906,960 Lehr 4,432,372 Monroe 4,456,013 DeRossi et al Published European Anderson et al Patent Application 0 080 347 to ______________________________________
In one embodiment of the apparatus of the present invention, changes in impedance between two spaced apart electrodes in the right ventricle can be used to determine ejection time as the blood content in the right ventricle changes during heart pumping activity. Then the changes in ejection time can be used to adjust the pacing rate.
In the article entitled "Continuous Measurement of Ventricular Stroke Volume by Electrical Impedance" by Geddes, Hoff, Mello and Palmer appearing in Vol. 4, No. 4 April-June 1966 issue of "Cardiovascular Research Center Bulletin" different techniques for measuring impedance in ventricles, particularly the left ventricle, for use in measuring ventricular stroke volume, are described.
Moreover, it has been proposed in the Geddes et al U.S. Pat. No. 4,291,699 to measure impedance by means of two electrodes in a right ventricle when electrical analysis indicates that ventricular fibrillation is present and persists. The impedance measurements are then used to determine mechanical pumping activity of the heart. A defibrillator is actuated only when both mechanical and electrical activity of the ventricle indicate a need for defibrillation.
The apparatus of the present invention differs from the previously proposed apparatus and methods by providing an implanted artificial pacemaker having a microprocessor therein with a program or algorithm stored in the microprocessor for controlling the pacing rate of pacing pulses supplied to an electrode at the end of a pacing lead in the right ventricle relative to, and in response to, the changes in the ejection time of the right ventricle as obtained by sensing the opening and closing of a pulmonary valve with sensing means mounted on the pacing lead and located in the right ventricle.